Medical ultrasound scanners typically produce a two-dimensional image of a planar region of a patient's body. The image is created by transmitting a series of ultrasound pulses into the region under investigation, and receiving and processing the echoes of the transmitted pulses, to build up a two-dimensional image. Modern ultrasound scanners are capable of obtaining images at a fast enough rate, e.g., 20 or more times per second, so that a real time display of the region under investigation can be created on a video monitor.
In recent years, ultrasound devices have been improved by adding the capability of obtaining Doppler information from a selected sample volume within the two-dimensional region under investigation. The Doppler data represents the velocity of structures within the selected volume. Thus if the selected volume is within a blood vessel, data can be obtained concerning blood velocity. Such data can be extremely useful in diagnosing various cardiac and other circulatory system problems.
Typically, the Doppler data is produced in both video and audio forms. The video form of the Doppler data consists of a two-dimensional graph on a video display, the graph having time along the horizontal axis and frequency along the vertical axis. Each column of the graph represents the frequency spectrum of the Doppler signal at the corresponding point in time. Such a display is typically scrolled across the screen as new data is acquired. For audio output, the raw Doppler signal, having a frequency equal to the transmitted frequency plus the Doppler modulation, is demodulated by removing the frequency of the transmitted signal, and the demodulated Doppler signal is then converted by a speaker into a corresponding audio signal. For most clinically significant cases, the frequency of the audio signal is within the range of human hearing, and thus can be used directly by an operator to sense blood flow patterns and other movement within the subject's body. An operator carrying out an investigation with the ultrasound scanner will often rely heavily on the information presented via the audio Doppler signal, in order to seek regions in the circulatory system in which more detailed measurement or imaging may be of interest. It is, therefore, very important that the audio Doppler signal not be substantially distorted.
The current generation of medical ultrasound scanners can typically operate in a "simultaneous" mode in which both image and Doppler data are collected and displayed apparently simultaneously, in real time. The data collection cannot be literally simultaneous, because the transmit burst length, focusing, and steering patterns used for image acquisition are different from those used for Doppler processing. Thus inherent limitations of beamformer and scanhead design require that the acquisition of image and Doppler data be time multiplexed. The multiplexing rate is made sufficiently high so that the image and Doppler data appear to be acquired simultaneously.
For image data, the multiplexing reduces the frame rate, sometimes to the point at which motion in the image no longer appears continuous. For Doppler data, the effects of multiplexing depend upon the particular multiplexing technique employed. A fundamental difference between imaging and Doppler measurement is that with imaging, only one pulse is needed in each beam direction to collect data for a given frame. For Doppler processing, on the other hand, one must obtain samples over a substantial period of time, in order to accurately estimate the frequency content of the Doppler signal. Thus the imaging function of an instrument can be interrupted without interfering significantly with the measurement, while Doppler measurements must be continuous for longer periods of time to obtain accuracy.
One form of multiplexing currently in use in connection with pulsed (as opposed to CW) Doppler simply alternates between image and Doppler pulses. For a given Doppler sample volume depth within the subject, this technique reduces the Doppler pulse repetition frequency by a factor of at least two. As a result, the maximum Doppler frequency (and therefore the maximum velocity) that can be unambiguously detected is also reduced by a factor of two. If the velocity exceeds this reduced maximum, aliasing will occur, and misleading results will appear in both the video and audio outputs. Pulse-by-pulse switching between image and Doppler modes may degrade the frame rate and pulse repetition frequency by a factor somewhat greater than two, because dead time will generally be required in order to avoid interference between the image and Doppler echoes.
Because of the difficulties in the alternating approach described above, it has become common to time multiplex image and Doppler pulses using larger time slices. For example, a series of image pulses are transmitted, for a time period sufficient to produce a single complete frame of the image to be displayed. A series of Doppler pulses are then transmitted, at whatever pulse repetition frequency is appropriate for the sample volume depth at which the Doppler data is to be collected. This technique can often be used with only minimal impact on the Doppler video output. However for Doppler audio output, such a technique, if uncorrected, results in modulation of the audio output at the interruption rate. Therefore to furnish an acceptable audio output, it is necessary to fill in the Doppler data during the time intervals in which image data is being acquired.
Prior Doppler fill-in techniques are described in U.S. Pat. Nos. 4,407,293 and 4,559,952. The prior art techniques, in general, are based upon the generation of a substitute signal that replaces the directly measured Doppler signal for audio and/or video output, during periods of time when the direct Doppler signal is unavailable. The substitute signal preferably has spectral properties close to those of the actual Doppler signal, and also produces audible sound close to that of the actual signal when fed to an audio output device. However some techniques provided in the prior art have not been capable of providing Doppler fill-in without perceptibly altering the audio output produced by the ultrasound scanner. In addition, the time domain techniques used in the prior art have generally lacked the flexibility needed to tailor the fill-in to the characteristics of human hearing.